1. Technical Field
The present invention relates to a variable resistance nonvolatile storage element whose resistance value changes due to application of voltage pulses.
2. Background Art
In recent years, along with the progress of digital technology, electronic devices such as portable information devices and home information appliances have further become highly functional. As the functions of these electronic devices have become more sophisticated, miniaturized and high-speed semiconductor elements used in the electronic devices have been rapidly developed. Among the semiconductor elements, nonvolatile memories having a large capacity as represented by flash memories have been increasingly used for various purposes. Moreover, research and development of variable resistance nonvolatile memory devices using so-called variable resistance elements as next-generation new nonvolatile memories in place of the flash memories has been in progress. Here, a variable resistance element refers to an element that has characteristics that its resistance value reversibly changes due to electrical signals, and is further capable of storing information corresponding to the resistance value in a nonvolatile manner.
A nonvolatile storage element whose variable resistance layer is formed by stacking transition metal oxides having different oxygen content atomic percentages has been proposed as an example of the variable resistance element. It is disclosed that resistance change is stabilized by selectively causing oxidation and reduction reaction in an interface between a variable resistance layer having a high oxygen content atomic percentage and an electrode in contact with the variable resistance layer (see Patent Literature (PTL) 1, for instance).
FIG. 13 shows a variable resistance nonvolatile storage element 90 including a conventional variable resistance element 90a. A first line 101 is formed on a substrate 100, and a first interlayer insulating layer 102 is formed to cover the first line 100. A first contact plug 103 to be connected to the first line 101 is formed to penetrate the first interlayer insulating layer 102. The variable resistance element 90a that includes a second electrode (here, lower electrode) 104 on the first interlayer insulating layer 102, a variable resistance layer 105, and a first electrode (here, upper electrode) 106 is formed to cover the first contact plug 103. A second interlayer insulating layer 107 is formed to cover the variable resistance element 90a, and a second contact plug 108 that penetrates the second interlayer insulating layer 107 connects the first electrode 106 and a second line 109. The variable resistance layer 105 has a stack structure of a first transition metal oxide layer 105x and a second transition metal oxide layer 105y, and comprises transition metal oxides of the same type. A transition metal oxide comprised in the first transition metal oxide layer 105x has an oxygen content atomic percentage higher than that of a transition metal oxide comprised in the second transition metal oxide layer 105y. 
With such a structure, when a voltage is applied to the variable resistance element 90a, most of the voltage is applied to the first transition metal oxide layer 105x having the high oxygen content atomic percentage and indicating a higher resistance value. Moreover, oxygen that is capable of contributing to reaction is abundant near an interface between the first electrode 106 and the first transition metal oxide layer 105x. Thus, the oxidation and the reduction reaction selectively occur in the interface, thereby stabilizing resistance change.